1. Field of the Invention
This invention relates to a process of shaping the edge of a combustion chamber recess in the piston head of a finish-machined light-alloy piston.
2. Discussion of Prior Art
Thermally induced cracks at right angles to the rim of the combustion chamber recess may form in light-alloy pistons having a combustion chamber recess in the piston head when the piston material is subjected to a stress in excess of its highest permissible stress.
Various measures can be adopted to avoid such cracks. For instance, it is known to provide the piston adjacent to the rim of the combustion chamber recess with inserts made of heat-resistant steel or of cermets. But these measures have the disadvantage of appreciably adding to the weight of the piston. Additionally, the material of the inserts has a relatively poor thermal conductivity and a different coefficient of expansion so that the surrounding light alloy may suffer damage due to heat. The same disadvantages are inherent in inserts made from more ductile materials which are secured adjacent to the rim of the recess by electron beam welding or friction welding. Much better results can be obtained by a partial anodizing of the piston head and of the rim portion of the combustion chamber recess (German Pat. No. 2,507,899).
German Offenlegungsschrift No. 2,154,595 describes a process in which the piston blank is made to have a machining allowance adjacent to the rim of the combustion chamber recess and a zone which is disposed outside the portion added for the machining allowance, adjacent to the subsequently formed rim of the combustion chamber recess, is refined in that it is remelted by means of a charge carrier beam. The portion added for the machining allowance is then removed to expose the remelted zone. The remelting by means of a charge carrier beam results in a melting of a relatively small quantity of material close to the beam and that material solidifies as soon as the charge carrier beam has been removed. This step results in a fine-grained structure, which is relatively strong and has a large elongation.
It is known from "Motortechnische Zeitschrift" 40(1979) pages 261 to 266, particularly page 164, that good results have been obtained sometimes and no improvement has been obtained in other cases with such piston having a combustion chamber recess remelted by means of a charge carrier beam so that the risk of cracks forming at the rim of the combustion chamber recess cannot be avoided with absolute safety.
For this reason it is an object of the present invention reliably to prevent the formation of thermally induced cracks adjacent to the rim of the combustion chamber recess of a light-alloy piston of the kind described first hereinbefore.